Symantc Email Proxy

A few of my clients are running a in-house developed application that sends out emails. On some of the clients, seemingly at random, these emails are being blocked by SEP 11 with the following message: as in first instance it say the mail server rejected the message......

The picture is showing the settings / properties of the application, where in I've changed the server name with it's IP address, seems OK, as this time I haven't been denied from Server, but still the client wasn't able to get the message.

In second option I've disabled the SEP on the client but NO LUCK. In my next step I've removed the SEP completely from client and GUESS WHAT it works and the recepient ha received the message.

So, after this I reinstalled the SEP (client) then start getting the message as "unable to access the temporary file folder"

THIS IS THE FIRST TIME I'VE EVER JOINED THE FORUM POSTED MY PROBLEM, ANY ADVISE....